1. Field of the Invention
The present invention relates to a magnetic material and a process for preparing the same. More particularly, the present invention is concerned with a magnetic material comprising a rare earth element, iron, nitrogen, hydrogen and oxygen, which exhibits excellent magnetic properties, such as high residual magnetization, intrinsic coercive force and loop rectangularity as well as high magnetic anisotropy, which excellent magnetic properties have not been experienced with conventional non-sintered or non-casted magnetic materials, and is also concerned with a process for preparing the same. The magnetic material having such excellent magnetic properties can advantageously be prepared in a particulate form and therefore, the magnetic material particles of the present invention can be bonded by means of a binder to obtain a so-called bonded magnet having a desired shape and maintaining the excellent magnetic properties of the magnetic material used, differing from the conventional magnetic material which, in producing a bonded magnet therefrom, needs to be fabricated into a sintered form (which exhibits maximum magnetic properties) and then pulverized to particles followed by bonding by means of a binder, in which the pulverization step inevitably causes the magnetic properties of the final bonded magnet to be lowered. The present invention is also concerned with a bonded magnet which advantageously maintains the excellent magnetic properties of the magnetic material used
2. Description of the Prior Art
Magnetic materials are widely employed, and insofar as hard magnetic materials for small actuators are concerned, there is an increased demand for smaller and stronger magnets. It has conventionally been known that rare earth magnetic materials, such as samarium-cobalt (Sm-Co) and neodymium-iron-boron (Nd-Fe-B), exhibit maximum magnetic properties when being fabricated into a sintered magnet, due to the mechanism for exerting magnetic properties Accordingly, the magnetic materials of Sm-Co and Nd-Fe-B are individually fabricated into sintered magnets and then, ground or cut into desired shapes for use as magnetic parts 0n the other hand, there are known bonded magnets comprised of magnetic material particles bonded together by means of a binder, which bonded magnets can advantageously be prepared in any desired shape by injection molding or compression molding of magnetic material particles without requiring grinding or cutting as in the case of sintered magnets However, when the conventional rare earth magnetic materials are employed in producing bonded magnets, it is necessary to prepare magnetic particles having a particle size as small as from about 10 to about 50 .mu.m by pulverizing sintered magnets having been produced from the magnetic materials and then subjecting the obtained magnetic particles to molding. Therefore, not only a lowering of magnetic properties during the pulverization of the sintered magnet but also an increase in cost due to the requirement of such an additional step, is unavoidable.
In these situations, developing of magnetic materials in a particulate form having a small particle size, e.g., less than 10 .mu.m and exhibiting high magnetic properties has been desired in the art so that bonded magnets having satisfactory magnetic properties can be obtained.
The present inventors previously proposed a magnetic material represented by the formula: EQU R.sub..alpha. Fe.sub.(100-.alpha.-.beta.-.gamma.) N.sub..beta. H.sub..gamma.
or EQU R.sub..alpha. Fe.sub.(100-.alpha.-.beta.-.gamma.-.delta.) N.sub..beta. H.sub..gamma. M.sub..delta.
wherein
R is at least one rare earth element inclusive of Y, PA1 M is at least one additive selected from the group consisting of Sn, Ga, In, Bi, Pb, Zn, Al, Zr, Cu, Mo, Ti, Si, MgO, Al.sub.2 O.sub.3, Sm.sub.2 O.sub.3, AlF.sub.3, ZnF.sub.2, SiC, TiC, AlN and Si.sub.3 N.sub.4, PA1 .alpha. is from 5 to 20 atomic percent, PA1 .beta. is from 5 to 30 atomic percent, PA1 .gamma. is from 0.01 to 10 atomic percent and PA1 .delta. is from 0.1 to 40 atomic percent
(see European Patent Application Publication No. 0 369 097Al). This proposal was made, based on the novel finding that the nitriding and hydriding of a pulverized alloy containing a rare earth metal and iron and the incorporation of a specific metal species and/or specific inorganic compound which is defined as M in the above formula can provide particulate magnetic materials having improved magnetic properties. These particulate magnetic materials can be not only fabricated into sintered magnets, but also can be bonded together by means of a binder to provide bonded magnets. In preparing the above-mentioned system of magnetic materials, fine pulverization, after the nitriding and hydriding, of the coarsely pulverized materials must be conducted in an atmosphere of inert gas so that inclusion of oxygen into the material is prevented.
The present inventors have made further investigations on the above-mentioned nitride-hydride type of magnetic materials in order to further improve the magnetic properties thereof. As a result, it has unexpectedly been found that when the fine pulverization of the material after being nitrided and hydrided is performed in an organic medium having an extremely limited amount of oxygen dissolved therein or in an oxygen-containing inert gas atmosphere having an extremely limited partial pressure of oxygen, a magnetic material having remarkably improved magnetic properties can be obtained. This is surprising in view of a common knowledge in the art such that inclusion of oxygen into a magnetic material as well as oxidation of a magnetic material must be prevented. The present invention has been completed, based on such an unexpected novel finding.